Below Paris, there is a vast hot water reserve at 1700m underground. A company is already using it (and other heat sources) to provide urban heating to houses and buildings in Paris, including private houses.

There was a great effort to develop this use following the oil crisis in the seventies, which subsided afterwards and have been renewed since 2007.

Regarding the effective development of thermal insulation, constraints are often ineffective -except for new buildings- because the owners who are supposed to pay for the works are simply unable to do so. A lot of the collective houses in the greater Paris are simply broke and cannot even pay for the security maintenance of the buildings. Problems are frequent.

A new strategy has been initiated by the region Ile de France, which is to develop a system of third party investor: a public owned company (but of private law) is acting as a financer, and contracting authority in place of the owners organisation. It collects every aids and financial incentives already available for the project and controls the works. It pays for all works and then gets paid through a share of the savings created by the energy overhaul, while the owners of the building have to pay for their (reduced) energy use and diverting part of the saving to the third party company to pay back for the works.

I tried to contact them without success, as it's quite new and probably not completely operational.